Confusing diagnostic when using closures

[StyMaar]

When trying to compile this code (Rust stable 1.49.0)

fn my_function(callback: impl FnMut(&str) + Send + 'static){}

fn main(){
    let closure_consumer = |data| {};
    my_function(closure_consumer);
}

I expected it to compile fine,

Instead, we get this funny nonsensical error :

  |
  |     my_function(closure_consumer);
  |     ^^^^^^^^^^^ one type is more general than the other
  |
  = note: expected type `FnOnce<(&str,)>`
             found type `FnOnce<(&str,)>`

The code compiles fine if we add a type annotation to the closure (notice the |data: &str|)

fn my_function(callback: impl FnMut(&str) + Send + 'static){}

fn main(){
    let closure_consumer = |data: &str| {};
    my_function(closure_consumer);
}

This may be a type inference bug (because closure are supposed to work even without type annotations) but it's also (and most importantly IMHO) a diagnostic bug, because the error gives me zero information about what is going on.

When compiling with the latest nightly (1.51.0-nightly (2021-01-28 c0b64d9)), we have a more detailed error, but it doesn't make a better job at explaining what we are supposed to do to make this code run :

  |
6 |     my_function(closure_consumer);
  |     ^^^^^^^^^^^ lifetime mismatch
  |
  = note: expected type `FnOnce<(&str,)>`
             found type `FnOnce<(&str,)>`
note: this closure does not fulfill the lifetime requirements
 --> src/main.rs:5:28
  |
5 |     let closure_consumer = |data| {};
  |                            ^^^^^^^^^
note: the lifetime requirement is introduced here
 --> src/main.rs:2:31
  |
2 | fn my_function(callback: impl FnMut(&str) + Send + 'static){}

[mbrubeck]

See also #75791.

[subtle-byte]

Adding the second argument to the closure

fn my_function(_callback: impl FnMut(&mut i32, &mut i32)) {}

fn main() {
    let closure_consumer = |_a: &mut i32, _b| {};
    my_function(closure_consumer);
}

(playground)

makes the error message even more strange:

error[E0308]: mismatched types
 --> src/main.rs:5:5
  |
5 |     my_function(closure_consumer);
  |     ^^^^^^^^^^^ one type is more general than the other
  |
  = note: expected type `FnOnce<(&mut i32, &mut i32)>`
             found type `for<'r> FnOnce<(&'r mut i32, &mut i32)>`

It seems like the problem is with the first argument, but indeed adding the explicit type to the second argument (i.e. _b: &mut i32) removes the error.

[whatisaphone]

You can work around it by partially specifying the type:

fn foo(f: impl Fn(&i32)) {}

fn main() {
    let broken = |_| {};
    foo(broken);

    let works = |_: &_| {};
    foo(works);
}

[estebank]

Current output:

error[E0308]: mismatched types
 --> src/main.rs:6:5
  |
6 |     my_function(closure_consumer);
  |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ one type is more general than the other
  |
  = note: expected trait `for<'a> FnMut<(&'a str,)>`
             found trait `FnMut<(&str,)>`
note: this closure does not fulfill the lifetime requirements
 --> src/main.rs:5:28
  |
5 |     let closure_consumer = |data| {};
  |                            ^^^^^^
note: the lifetime requirement is introduced here
 --> src/main.rs:2:31
  |
2 | fn my_function(callback: impl FnMut(&str) + Send + 'static){}
  |                               ^^^^^^^^^^^

error: implementation of `FnOnce` is not general enough
 --> src/main.rs:6:5
  |
6 |     my_function(closure_consumer);
  |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ implementation of `FnOnce` is not general enough
  |
  = note: closure with signature `fn(&'2 str)` must implement `FnOnce<(&'1 str,)>`, for any lifetime `'1`...
  = note: ...but it actually implements `FnOnce<(&'2 str,)>`, for some specific lifetime `'2`

error[E0308]: mismatched types
 --> src/main.rs:5:5
  |
5 |     my_function(closure_consumer);
  |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ one type is more general than the other
  |
  = note: expected trait `for<'a, 'b> FnMut<(&'a mut i32, &'b mut i32)>`
             found trait `for<'a> FnMut<(&'a mut i32, &mut i32)>`
note: this closure does not fulfill the lifetime requirements
 --> src/main.rs:4:28
  |
4 |     let closure_consumer = |_a: &mut i32, _b| {};
  |                            ^^^^^^^^^^^^^^^^^^
note: the lifetime requirement is introduced here
 --> src/main.rs:1:32
  |
1 | fn my_function(_callback: impl FnMut(&mut i32, &mut i32)) {}
  |                                ^^^^^^^^^^^^^^^^^^^^^^^^^

error[E0308]: mismatched types
 --> src/main.rs:5:5
  |
5 |     my_function(closure_consumer);
  |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ one type is more general than the other
  |
  = note: expected trait `for<'a, 'b> FnOnce<(&'a mut i32, &'b mut i32)>`
             found trait `for<'a> FnOnce<(&'a mut i32, &mut i32)>`
note: this closure does not fulfill the lifetime requirements
 --> src/main.rs:4:28
  |
4 |     let closure_consumer = |_a: &mut i32, _b| {};
  |                            ^^^^^^^^^^^^^^^^^^
note: the lifetime requirement is introduced here
 --> src/main.rs:1:32
  |
1 | fn my_function(_callback: impl FnMut(&mut i32, &mut i32)) {}
  |                                ^^^^^^^^^^^^^^^^^^^^^^^^^